1. Field of the Invention
This invention relates generally to the art of semiconductor devices. More specifically, the invention relates to manufacture of Organic Light Emitting Diode based displays and other active electronic devices.
2. Related Art
Display and lighting systems based on LEDs (Light Emitting Diodes) have a variety of applications. Such display and lighting systems are designed by arranging a plurality of photo-electronic elements (“elements”) such as rows of individual LEDs. LEDs that are based upon semiconductor technology have traditionally used inorganic materials, but recently, the organic LED (“OLED”) has come into vogue for certain applications. Examples of other elements/devices using organic materials include organic solar cells, organic transistors, organic detectors, and organic lasers.
The OLED is typically comprised of two or more thin organic layers (e.g., an electrically conducting polymer layer and an emissive polymer layer where the emissive polymer layer emits light) which separate an anode and a cathode. Under an applied forward potential, the anode injects holes into the conducting polymer layer, while the cathode injects electrons into the emissive polymer layer. The injected holes and electrons each migrate toward the oppositely charged electrode and produce an electro-luminescent emission upon recombination in the emissive polymer layer.
Each of the OLEDs can be a pixel element in a passive or active matrix OLED display. Such pixels can be arranged in a row-column fashion and would be addressed and switched on/off differently depending upon whether the display was active or passive matrix. In the passive matrix case, each pixel is not individually controlled, but rather an entire row of pixels is biased with respect to the intersecting column. When the difference between the voltage applied to the column (e.g. the anode) and the row (e.g. the cathode) is greater than the turn-on voltage, the pixel element at the intersection of the row and the column is illuminated. One obstacle in manufacturing of organic light emitting diodes is the high leakage current, which is defined as the amount of current flowing in the reverse direction when a forward bias is applied to a pixel. A high leakage current results in undesirable cross talk, i.e. illumination of the neighboring pixels when a particular pixel is addressed. While the exact nature of the cause of leakage current is not well-determined, in general, leakage current is caused by the presence of undesired conductive pathways formed during the fabrication process between the conductive polymer or the anode on one hand and the cathode materials on the other hand. The formation of the conductive pathways may simply be due to presence of a small hole in the emissive layer or due to the material build up against a photoresist edge during deposition or coating.
FIG. 1 illustrates a typical OLED-based passive matrix display during its manufacture. FIG. 1 is a side perspective view of a passive matrix OLED display 100 midway in the manufacturing process. Display 100 includes a patterned anode layer 102 (typically the columns) that are patterned on top of a glass substrate 101. Anode layer 102 is typically composed of a metal-oxide compound such as ITO (Indium Tin Oxide). After anode patterning (usually via a photolithography and etching step), metal lines (not shown) are deposited and patterned upon the anode pattern using methods known in the art (e.g. metal deposition, photolithography and etch).
Thereafter, cathode separators 110 are formed upon the surface of the substrate perpendicular to the ITO strips. These separators are typically photoresist layers that are patterned as shown using a photolithography technique. One of the purposes of the separators is to provide electrical separation of the individual rows of the top electrode layer, namely the cathode layer. FIG. 1 depicts a metal cathode layer 104 that is deposited by, for example, thermal evaporation, on top of various polymer layers (such as an emissive polymer layer 109 and a conducting polymer layer 108), to provide a complete conductive pathway for activating pixels which are subsequently formed. The intersection of cathode layer 104 and anode layer 102 creates a matrix of active pixels such as pixel 106 (shown with diagonal shading). The pixel 106 illuminates under an application of voltage which is forward biased as discussed above.
Long term exposure of OLED pixels to temperature, moisture and/or mechanical stress results in OLED degradation and malfunctioning, particularly is because of the polymer layers and the thin-film nature of the final assembly. Efficient OLED devices generally require the use of low work function materials for electron injection. These materials are typically metals such as Mg, Ca, Li, Ba, or metal halides such as LiF or CsF, which readily react with oxygen and water. A low work function calcium cathode, for example, survives only a short time in air due to rapid device degradation caused by atmospheric moisture and oxygen. Such highly reactive material can also undergo chemical reactions with the nearby organic materials present within the device, which can also have negative effects on the device. To protect the OLED pixels from environmental and mechanical damage, a cap 120 encapsulates the display 100 (usually over the substrate, shown but not enumerated). The encapsulation is in some instances performed by placing the cap 120 over a layer of epoxy (not shown) which covers the display 100. In other instances, the epoxy contains spacer particles which are used to separate the cap 120 from touching certain portions or all of the display 100. The shape of the cap 120 shown in FIG. 1 is merely illustrative, and may be any desired shape or form. The encapsulation cap 120 may be composed of a variety of materials, including but not limited to glass, ceramics, plastic or metals. The encapsulation technique may be mechanical or chemical or a combination thereof. The process of encapsulation is ordinarily carried out in a “glove box” environment. Often, a nitrogen gas (N2) is used during encapsulation such that the gas is trapped under the encapsulating cap and fills the volume between the encapsulating cap and the display 100. Other gases have also been used for encapsulation. For example in U.S. Pat. No. 6,104,137 issued to Abiko et al. a method is disclosed wherein the OLED is encapsulated under oxygen, the purpose of which is to improve the leakage current in the OLED. Oxygen apparently reduces the prominence of leakage paths, but can also create undesirable effects such as adverse chemical reactions with the light emitting polymer and requires careful handling in a fabrication environment. Furthermore, oxygen is expensive and increases manufacturing costs.
Thus, it is highly desirable to provide alternative methods to reduce leakage current.